It’s simple and cheap keeping pollen beetle out of oilseed rape, isn’t it? Just add a pyrethroid insecticide to the tank when you apply your fungicide at green-yellow bud stage, then you needn’t give it a second thought.
But if you do that, it may not be simple and cheap for long, warns Sam Cook of Rothamsted Research. Prophylactic spraying incurs a high risk, as German oilseed rape growers know to their cost. There, an estimated 30,000ha was completely lost to the pollen beetle in 2006 when pyrethroids failed to control the pest, costing them up to €25m.
In just a few years, pollen beetle populations in mainland Europe developed widespread resistance to the pyrethroids. Now, for many growers the only option is to use chemicals from a different group, and they are more costly. The neonicotinoid, Biscaya (thiacloprid) costs three times the price of a typical pyrethroid such as Hallmark Zeon (lambda-cyhalothrin).
If UK growers continue to use pyrethroids as a routine – according to DEFRA statistics, the average crop receives at least two insecticides, mostly pyrethroids, and a third of those hit pollen beetle – they are asking for trouble, warns Dr Cook.
There’s no justification for applying a pyrethroid between green and yellow bud – the critical risk period for pollen beetle damage – unless you can find on average 15 beetles per plant (five in backward crops) across the field, she stresses. Monitoring from the crop edges alone is poor practice since there are likely to be higher levels on the margins. Once in flower, spraying is unnecessary as plants are no longer vulnerable to damage.
Fortunately there are currently only limited populations in the UK that show pyrethroid resistance, most being in the eastern counties. But unpredictable weather brings uncertainty, so to counteract this threat, the HGCA has invested £130,000 of a total £750,000 in developing an integrated control strategy. Monitoring, risk assessment and crop management are being evaluated and developed further.
At present there is no monitoring trap commercially available for pollen beetle, however Dr Cook is confident the one designed for the project can be improved then marketed in the not too distant future. She first needs to be sure it is as efficient as possible and sited for maximum catch.
“Trapping will give us much needed data about pest behaviour. We also want to reduce the catch of non-target pests, so it’s important to get the right colour and bait with the most appealing synthetic brassica extract.”
The beetle prefers yellow but so do other insects, so responses to different shades of yellow will be determined and used if appropriate.
Trap and weather records will be used to modify a risk assessment program called proPlant, used widely on the continent. Once calibrated for the UK, it should offer a more accurate picture of the field situation than is currently available. This should allow growers to minimise variable costs and environmental impact, hopes Dr Cook.
• Project no. 3394: Development of an integrated pest management strategy for control of pollen beetle in winter oilseed rape; Rothamsted Research, Imperial College London, AICC, Bayer CropScience, Oecos, proPlant, Syngenta, Elsoms Seeds, Limagrain UK, KWS, Monsanto, Saaten Union, VSN International, DEFRA-CRD under a Sustainable Arable LINK; from March 2008 to March 2012.
• Oilseed rape too often routinely sprayed against pollen beetle
• Need to prolong life of pyrethroid insecticides and minimise environmental impact
• Develop traps and risk assessment programmes to alert growers to potential problem
• Evaluate non-chemical control measures
• Increasing our knowledge of pollen beetle behaviour and improving risk assessment should allow growers to prolong the life of the pyrethroid insecticides, and thus keep oilseed rape sustainable. The use of field traps must be cost-effective to be valuable.
• Interim report available on the HGCA website
Contributing to research
• If you’d be willing to place pollen beetle traps in your oilseed rape crop for six weeks this spring, you would not only help with this project but also in combating insecticide resistance. Trap kits and simple instructions are available from Sam Cook